A marathon negotiating session between the San Francisco Bay Area's main commuter train agency and its unions has stretched into a second day.

The contract talks between Bay Area Rapid Transit management and its two largest unions began around 10 a.m. Wednesday and were still going on early Thursday morning, more than 18 hours later. BART trains, meanwhile, were running on a normal schedule.

Federal mediator George Cohen said in an e-mail announcing that trains would continue running on Thursday morning that the two sides were making progress.

The contentious labor talks have dragged on for six months. BART workers walked off the job for nearly five days in July. That strike resulted in traffic jams and long lines for buses.

BART is the nation's fifth-largest rail system. It serves about 400,000 riders each weekday.